Event::add

说明

Marks event pending. Non-pending event will never occur, and the event
callback will never be called. In conjuction with
Event::del()
an event could be re-scheduled by user at any time.

If
Event::add()
is called on an already pending event, libevent will leave it pending and
re-schedule it with the given timeout(if specified). If in this case
timeout is not specified,
Event::add()
has no effect.